The purpose of these studies is to establish a better understanding of the energy metabolism in tissues, in vivo. Towards this goal, the laboratory concentrates on the use of non-invasive and non-destructive techniques to evaluate the biochemical and physiological function of the heart and skeletal muscle with regard to energy metabolism. The following major findings were made over the last year: 1) An "optical window" in the reflection spectrum (520 to 590nm) of the intact porcine, in vivo, was found which permitted the direct monitoring of oxygen tension in the cytosol, via myoglobin, and with in the mitochondria, using cytochrome c. Theoretical and in vitro experiments provided an physical basis for the measurement confirming the lack of interference from vascular hemoglobin. 2) Using this technique the effects of increased work on myocardial oxygenation was evaluated. It was found that the increase in blood flow associated with an increase in myocardial work maintained the epicardial cellular oxygen tension constant. These results suggest that tissue hypoxia is not involved in the regulation of epicardial coronary blood flow. 3) A new technique of correcting for light scattering effects in fluorometric determinations of mitochondrial NADH. This approach was critical in determining the effects of calcium on mitochondrial energetics. 4) Using 31P NMR and a specially constructed exercise device, the energy efficiency human skeletal muscle was determined in concentric, eccentric and isometric muscle action. The mechanochemical efficency (i.e. the amount of work generated by a given amount of ATP) of these processes was determined to be 15% in concentric activity and 35% in eccentric muscle action. 5) A normalization process to determine the metabolic stress of a given exercise protocol was developed. This normalization is based on the total muscle mass determined from 3-dimensional MRI studies. It was shown that this procedure, based on absolute muscle volume, was superior to more subjective measures of maximum voluntary contraction in a group of human subjects.